A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus, many moving parts may be provided. Movements may for example be performed by a substrate stage, a patterning device stage (e.g. a mask stage), cooling devices, etc. These movements may result in vibrations or other disturbances which may act on a projection system of the lithographic apparatus (also referred to as projection lens or lens). Thereby, vibrations or other disturbances of the projection system as a whole, and/or of optical elements thereof, may occur. Previously, an active lens mount has been devised in order to reduce an effect of such vibrations. In such active lens mount, a first piezo element is provided to exert a force on the projection system, and a second piezo element to measure the force. Using a suitable control system, a resulting force on the projection system may be reduced.